Among the nitrogen-containing compounds guanidine [], an amidine [], an aliphatic primary amine [], — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

Guanidine is an exceptionally strong organic base. Upon protonation at the $sp^2$ hybridized imine nitrogen ($=NH$), the resulting conjugate acid (guanidinium ion) is highly stabilized by the formation of three completely equivalent and indistinguishable resonance structures. This massive thermodynamic stabilization of the protonated form drives the equilibrium heavily toward proton acceptance.